But in general, the idea is that in order for something to boil, enough energy must be absorbed by it to cause vibrations large enough to enhance the kinetic energy of each molecule to the point where they break away from the coulombic forces that hold them together in the original liquid.
When a more massive solid is dissolved into such a liquid, the energy goes into not only exciting each water molecule to a higher kinetic energy but also each salt molecule to a higher kinetic energy. The more massive salt molecules themselves need a larger contribution of energy in order to excite them. This takes away from the contribution to the water molecules. The increased density ends up requiring more energy for the same amount of excitation that would lead to the boiling of the water.
Put another way, the salt molecules weigh down the water molecules and restrict their movement. This perhaps is easier to visualize.
Using the same logic, you may be able to see that the salt will lower the freezing point as well. And this is why salt (or even better, calcium chloride, an even heavier compound) is used on snow or ice to force it from a solid state back into a liquid state. The salt is an additional impediment to the water molecules' easy organization, so just a small amount of energy is needed to break that solid crystal organization up.
Answered by: Ted Pavlic, B.S., Electrical Engineer, The Ohio State University
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'One thing I have learned in a long life: that all our science, measured against reality, is primitive and childlike - and yet it is the most precious thing we have.'